I actually GMed the wormsign campaign three times now, and I tried to hide some difficulties on my first. The players were frustrated, then just waited to seeif the earned momentum to see if they won. I really did not like it.
I now just tell them openly what the difficulty is on every test. Sometimes they do not know why I want them to make a test, but they still can guess if the result is important or not. On the opposite, they can’t chose whether to use their momentum or various talents if they do not know the exact number of the difficulty.
Another way I thought about but did not try, is to ask them if they want to spend a momentum to know the exact difficulty, as the answer of a question. It might be interresting to do it that way.